1. Field of the Invention
The present invention relates to the preparation and growth of small size particles Mn2+ and alkali doped lanthanum aluminate phosphor by solid state and sol-gel methods. More specifically, the present invention provides green emitting Mn2+ and alkali doped lanthanum aluminate phosphor and process by thermally decomposing salts of lanthanum, manganese, alkali and alumina or sol-gel powders.
2. Description of the Related Art
The plasma display panel (PDP) as a medium of large format (60+″) television (TV), particularly high definition TVs (HDTV's) is gaining attention over cathode ray tube (CRT) based TVs due to its' high performance and scalability. Although, CRT works with less power and having better picture quality, it has size limitation. Larger screens (CRT) of diagonal size more than 40 inches have larger depth and very heavy. Conversely, diagonal size of PDP is growing day by day, as there is no problem with depth and weight.
The structure of a PDP, which is known in the art, is described in FIGS. 1a and 1b. FIGS. 1a and 1b represents the cross section of an AC PDP. The plasma display has of two large area glass substrates 11, 16. Front plate 11 is made with sustain electrode 12 and scanning electrode 13, covered with thick dielectric layer 14 and a thin protection layer (MgO) 15. Back plate 16 is made with address electrodes 17, reflective layer 18, barrier ribs 19 and red phosphor 20R (Y,Gd)BO3:Eu2+, green phosphor 20G ZnSiO4:Mn2+ (P1) or the blend of ZnSiO4:Mn2+ and Y,GdBO3:Tb3+, blue phosphor 20B BaMgAl10O17:Eu2+ coated by screen printing or ink jet process. Both the glass plates are frit sealed together and filed the space 21 with Xe, Ne gas mixture. When voltage isapplied, a discharge is developed in the space 21 producing Vacuum UV (147 and 173 nm). When phosphors 2ORGB are excited by VUV photons, they emit respective visible radiations viewed through the transparent front plate as an image 22.
The luminous efficiency of a PDP depends upon various factors including materials such as phosphors, gas mixture, dielectric layer, reflective layer, black matrix, electrodes, cell dimension and shape, nature, size and shape of electrodes, address waveforms, operating voltages, etc. The performance and lifetime of a PDP is strongly related to the nature of phosphors and their resistance to energetic discharge ions, electrons, and solarization from VUV arising from Xe/Ne gas discharge. Compared to standard emissive display such as CRTs (5–6 Im/W), the efficiency of a PDP is low (1–2 Im/W).
To improve the overall efficiency of PDPs, considerable developments related to materials, design, process and electronics are under way. Efforts are also being made to develop new phosphors as well as to improve existing phosphors. Due to vacuum UV specific wavelengths available from Xe discharge (147 nm and 173 nm), only a limited number of lamp phosphors are suitable for PDP applications. In addition to high luminous efficiency, PDP phosphors should have longer life or stability, required persistence, suitable color coordinates, color temperature, and color saturation.
The main application of large area plasma displays will be HDTV and high information content presentation. HDTV and similar type of display devices should have phosphors with low dielectric constant, required decay time, high resolution and high brightness for high performance. Screens coated in a close rib structure or closed cell structure with small particles exhibit higher packing density and also need lesser binder content.
Persistence value, which is defined as being 10% of the initial brightness, is another concern in selecting a phosphor, and should be between 4 and 9 ms, also. The three phosphors (red, green and blue) currently used in PDP's have different dielectric constants and particle morphology. Due to their physical nature, all of the three phosphors need different rheology of phosphor paste as well as different screening processes. In PDP applications these phosphors exhibit different electrical characteristics in a finished panel. This results in compromises in the performance of the display.
HDTV and similar type of devices should have high resolution and higher brightness for better performance. This can be achieved only with thin phosphor screens formed with very small phosphor particles (1–5 microns) in a close rib structure particularly in the case of PDP's. Screens with small particles have a higher packing density and also require lower binder content. Manganese activated zinc silicate phosphor with or without terbium activated yttrium gadolinium borate is currently used in plasma display panels (PDP) as a green emitting component due to its availability and high quantum efficiency.
The higher dielectric constant of zinc silicate phosphor is of particular concern as it charges more than its' blue and red counterparts and this results in a higher sustainer voltage. When compared with red and blue emitting phosphors, zinc silicate phosphor also exhibits longer persistence, lower dielectric constant, negative discharge and faster saturation with the VUV flux. Another suitable green candidate, Tb activated yttrium gadolinium borate, which shows lower color purity is described in U.S. Pat. No. 6,004,481. As a trade off, PDP industry is adopting the blend of P1 and Tb activated borate. Efforts are being made to develop new phosphors to satisfying all requirements and replace existing Mn activated zinc silicate phosphor or the blend of silicate and borate.
Some other phosphor candidates based on alkali halide aluminates are being suggested in Phosphor Handbook. U.S. Pat. Nos. 4,085,351; 5,868,963; and 6,423,248 B1 disclose the application of manganese activated aluminate phosphor with either of calcium, strontium, barium, magnesium or zinc in a gaseous discharge light-emitting element. European Patent No. EP 0 908 502 A1 teaches the preparation of barium or strontium magnesium aluminate by firing respective oxides or carbonate in presence of flux (AlF3) at 1450° C. for 48 hours (total time). International Patent Application No. WO 98/37165 describes a method of making oxygen containing phosphor powder, which includes alkaline earth aluminates by spray techniques. European Patent No. EP 1 359 205 A1 describes the method of preparation of various green emitting phosphors has La, Mg, Zn aluminates with Tb, Mn as activators.
Other related aspects to such phosphors are described in U.S. Pat. Nos. 4,150,321; 5,989,455; and 6,222,312 B1; European Patent No. 0 697 453 A1; International Patent No. WO 98/37165 by Hampden-Smith Mark, et al.; and publications entitled (1) “Fluorescence in α-Al2O3-like materials of K, Ba, La activated with Eu2+ and Mn2+” by M. Tamatani, Jap. J. Applied Physics, Vol. 13, No. 6, June 1974 pp950–956; (2) “The behavior of phosphors with aluminate host lattices” by J. L. Sommerdijk and A. L. N. Stevels, Philips Tech. Review Vol. 37, No. 9/10, 1977 pp221–233; and (3) “Principal phosphor materials and their optical properties” by M. Tamatani in “Phosphor Handbook” edited by S. Shionoya and W. M. Yen, CRC Press (1999) pp. 153–176.